The present invention relates to the art of multi-channel power equalization in wavelength-division-multiplexed (WDM) optical networks. It finds particular application in conjunction with data transmission for local and wide area optical networks and will be described with particular reference thereto. It is to be appreciate, however, that the invention is also amenable to other applications wherein high-speed data transfer among multiple users and/or long-distance data transmissions are desirable.
In the implementation and development of local and wide area optical networks, data transmission systems, routing networks, and the like, it is desirable to provide high-capacity, high-speed, and/or long-distance data transmissions among multiple users. Two valuable technologies useful for achieving the aforementioned goals include wavelength-division-multiplexers (WDM) and erbium-doped fiber amplifiers (EDFA). In conventional WDM systems, many different wavelength channels are simultaneously transmitted along the optical fibers or data transmission lines. A typical system will carry 4-16 different wavelength data channels with wavelength spacings varying between 0.8 and 4.0 nm. This approach dramatically increases the capacity of the transmission system and permits wavelength dependent optical network routing. EDFA""s offer several advantages including high gain, low-additive noise, and fiber compatibility. Most relevant for WDM applications is the EDFA""s ability to amplify multiple wavelengths over a wide band width, typically 1 MHz. Additionally, for robust networks and/or systems, it is advantageous that power differentials between the various WDM channels remain small so as to minimize interchannel cross talk and ensure adequate gain for all the channels. Moreover, stability to dynamic changes in the system parameters is paramount to the successful implementation of a high-speed WDM optical network.
Traditionally, two obstacles in implementing WDM networks are: (1) the non-uniformity of EDFA gain, and (2) dynamic changes in channel powers. Since the EDFA gain is not uniform with wavelength, the lower gain channels progressively lose power relative to the higher gain channels. Amplifier chains impart a significant power and signal-to-noise ratio (SNR) differential among the various channels, significantly limiting the transmission distance and usable amplifier band width. Conventionally, the usable band width can be as small as approximately 5 nm after an EDFA cascade. In addition, as channel powers vary in a dynamic network, several system complications may arise which can potentially cause network failure. Variables which are troublesome include: changes in the input signal powers; drift in component wavelength selectivity; changes in link losses; and, changes in amplifier gain. Variable insertion losses, neighboring channel addition and deletion, unstable laser power, non-uniform EDFA gain, and microsecond long gain transients in EDFA cascades are other system parameters whose change can have deleterious effects on signal power.
In the past, passive channel power equalization methods such as long-period fiber grading, end-to-end telemetry, and the like have been employed to at least partially address the above-mentioned concerns. However, while passive gain equalization schemes can provide performance improvement for a point-to-point static or slow-changing link, they are not satisfactory for a dynamically reconfigurable network or for a static network with potential parameter changes. This is because the individual channel powers may vary significantly due to the dynamic and distributed characteristics of the WDM network and the EDFA gain non-uniformity varies with dynamic input load. Even an end-to-end telemetry technique, in which the output determines a wavelength selective attenuation of the input, is not adequate for systems that may change faster than tens of milliseconds. In another previous undertaking employed to address the above-mentioned concerns, erbium-doped fluoride fibers having a smaller gain non-uniformity were utilized. However, such a system cannot accommodate the dynamic changes in the network.
The present invention contemplates a new and improved dynamic network equalization module that overcomes the above-referenced problems and others. Moreover, it equalizes WDM channel powers to ensure robust network operation, a high gain, and a high SNR for all channels.
In accordance with one aspect of the present invention, a dynamic power equalization module for an optical network is provided. It includes a wavelength-division-multiplexer having an optical input for receiving an optical data transmission containing a plurality of different channels each with correspondingly different wavelengths. The wavelength-division-multiplexer spectrally separates the optical data transmission received by the optical input into parallel optical outputs which correspond to the different channels. A parallel array of acousto-optic modulators driven by RF acoustic signals is connected to the optical outputs of the wavelength-division-multiplexer. A coupler passively combines optical outputs from the parallel array of acousto-optic modulators such that the combined optical outputs from the parallel array of acousto-optic modulators is relayed to a plurality of coupler outputs corresponding to a feedback loop for the different channels and at least one coupler output corresponding to the dynamic power equalization module output. A plurality of optical filters are connected to the coupler outputs corresponding to the feedback loop. Each optical filter filters-out wavelengths that do not correspond to the channel in which they are being transmitted. A plurality photodetectors each receive an output from corresponding optical filters such that the plurality of photodetectors produce signals representative of powers of the different channels. A dynamic control circuit compares the signals produced by the plurality of photodetectors to determine the relative power of the different channels and generates the RF acoustic signals that drive the parallel array of acousto-optic modulators. The parallel array of acousto-optic modulators are driven such that each acousto-optic modulator dynamically controls each channels"" transmission and reduces its power level to be substantially the same as that of a reference channel.
In accordance with another aspect of the present invention, a dynamic power equalization module for an optical network includes a module input for receiving an optical transmission containing a combination of different channels. Each channel has a different characteristic wavelength and different varying powers. A power equalization device that is polarization insensitive is connected between the module input and a module output. The power equalization device is driven by RF acoustic control signals. A feedback loop which receives at least a portion of output from the module output and generates, based upon a comparison of relative powers of the different channels, the RF acoustic control signals which drive the power equalization device. The power equalization device is driven such that it dynamically controls transmission of the different channels to substantially equalize power differentials therebetween.
In accordance with a more limited aspect of the present invention, the power equalization device is a polarization independent acousto-optic tunable filter.
In accordance with a more limited aspect of the present invention, the power equalization device includes at least one acousto-optic tunable filter driven by the RF acoustic control signals from the feedback loop in a bar-state, wherein the acousto-optic tunable filter functions as a multi-channel notch filer.
In accordance with a more limited aspect of the present invention, the power equalization device includes at least one acousto-optic tunable filter driven by the RF acoustic control signals from the feedback loop in a cross-state, wherein the acousto-optic tunable filter functions as a multi-channel transmission filter.
In accordance with a more limited aspect of the present invention, the feedback loop includes, at the module output end, a splitter which separates the potion of the output received from the module output into individual channels. A plurality of optical filters, each optical filter connected in series with a corresponding channel, filter-out wavelengths that do not correspond to the characteristic wavelength of the channel. A plurality of photodetectors are connect to the optical filters such that each photodetector receives a corresponding channel. The photodetectors produce signals representative of the powers of the different channels. A dynamic control circuit is connected to the plurality of photodetectors such that the dynamic control circuit receives the signals produced by the photodetectors, compares the signals produced by the photodetectors to determine relative powers of the different channels, and generates the RF acoustic control signals that drive the power equalization device based on the dynamic control circuit""s comparison.
In accordance with another aspect of the present invention, a dynamic power control module for an optical network is provided. It includes a module input which receives an optical transmission containing multiple channels. Each channel has a different characteristic wavelength and power. A power control system receives the optical transmission from the module input. The power control system is driven by control signals to independently regulate the power of each channel. A module output receives the optical transmission from the power control system. A feedback loop samples the optical transmission from the module output. The feedback loop measures the power of each individual channel and in response to those measurements generates the control signals that drive the power control system. The power control system is driven so that the power of each channel at the module output is at a desired level.
In accordance with a more limited aspect of the present invention, the feedback loop includes a divider that spectrally separates the sampling of the optical transmission into individual channels which are received by a plurality of photodetectors such that the individual channels are each measured by a separate photodetector. Each photodetector generates an electrical signal representative of the power of the channel it is measuring.
In accordance with a more limited aspect of the present invention, the photodetectors are PIN detectors.
In accordance with a more limited aspect of the present invention, the feedback loop includes a scanning optical spectrometer for measuring the power of each individual channel.
In accordance with a more limited aspect of the present invention, an acousto-optic tunable filter functions as the scanning optical spectrometer.
In accordance with a more limited aspect of the present invention, the power control system includes a divider that spectrally separates the optical transmission from the module input into individual channels. An array of optical modulators driven by the control signals are arranged to receive the individual channels such that each channel is regulated by a separate optical modulator. A recombiner receives the regulated channels and recombines them.
In accordance with a more limited aspect of the present invention, the optical modulators are polarization insensitive.
In accordance with a more limited aspect of the present invention, the optical modulators are acousto-optic modulators and the control signals are RF acoustic signals.
In accordance with a more limited aspect of the present invention, the divider is a dense wavelength-division-multiplexer.
In accordance with a more limited aspect of the present invention, the power control system is a single device driven by the control signals such that it separately regulates each individual channel of the optical transmission received from the module input without spectrally dividing the optical transmission into the individual channels.
In accordance with a more limited aspect of the present invention, the single device is a polarization independent acousto-optic filter and the control signals are RF acoustic control signals.
In accordance with a more limited aspect of the present invention, the power of each channel at the module output is maintained at substantially the same level.
In accordance with a more limited aspect of the present invention, a power differential between channels at the module output is less than 1 dB.
In accordance with a more limited aspect of the present invention, a difference in wavelength between adjacent channels is less than 3 nm.
In accordance with a more limited aspect of the present invention, the dynamic power control module has a dynamic response time faster than 0.1 xcexcs.
In accordance with another aspect of the present invention, a method for dynamically equalizing power in an optical transmission having multiple channels is provided. It includes receiving the optical transmission having multiple channels. Each channel has a different characteristic wavelength and power. The optical transmission having the multiple channels is routed through a polarization insensitive power equalizer. Next, at least a portion of an output from the polarization insensitive power equalizer is taken and routed through a feedback loop. The power of the multiple channels in the feedback loop is dynamically compared relative to one another. RF acoustic control signals are generated in response to the comparison, and the power equalizer is driven with the RF acoustic control signals such that the multiple channels have substantially the same power.
In accordance with a more limited aspect of the present invention, the routing step of the method of includes spectrally separating the optical transmission by wavelength into an array of parallel transmissions such that each transmission corresponds to one of the multiple channels. The array of parallel transmissions are then routed through a parallel array of acousto-optic modulators driven by the RF acoustic control signals. The array of parallel transmissions is then recombined.
In accordance with a more limited aspect of the present invention, the dynamically comparing step of the method includes separating the portion routed through the feedback loop into multiple transmissions, each transmission including the multiple channels having different characteristic wavelengths. Each transmission is converted into separate channels by optically filtering out wavelengths from each transmission that do not correspond to the characteristic wavelength of the channel. Next, signals are generated representative of the powers of the channels, and the signals are dynamically compared relative to one another.
In accordance with a more limited aspect of the present invention, the routing step of the method includes routing the optical transmission through at least one acousto-optic tunable filter driven by the RF acoustic control signals such that the acousto-optic tunable filter functions as a notch filter in a bar-state.
In accordance with a more limited aspect of the present invention, the routing step of the method includes routing the optical transmission through at least one acousto-optic tunable filter driven by the RF acoustic control signals such that the acousto-optic tunable filter functions as a transmission filter in a cross-state.
One advantage of the present invention is that power differentials in a rapidly changing dynamic network are minimized.
Another advantage of the present invention is that changing system parameters are compensated for while maintaining uniformity of power across multiple channels.
Another advantage of the present invention is the high speed equalization of the powers across multiple channels.
Another advantage of the present invention is the flexibility of channel spacing across the wavelength spectrum.
Another advantage of the present invention is that it permits long-distance transmission while maintaining power equalization across multiple channels.
Another advantage of the present invention is that it compensates for gain non-uniformities associated with EDFA""s.
Another advantage of the present invention is improved signal to noise ratio characteristics for a multi-channel optical network.
Another advantage of the present invention is minimized interchannel cross talk and sufficient gain for all channels of a multi-channel optical network.
Another advantage of the present invention is its insensitivity to polarization.
Another advantage of the present invention is the suppression of amplified spontaneous emission (ASE) allowing for a longer channel propagation distance.
Still further advantages of the invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.